Hate to break it to you but that isn't a big cock. Below average girth giving the illusion of length on something thats altogether quite short. Bellend is average size also.
Your girlfriend has probably seen much bigger, statistically speaking.

I don't understand how these douchebags get women like this. It's like bizarro world where women like sucking dicks while being recorded and like being forced to give oral with a guy pushing her head into his penis. Seriously I don't get women.